The present invention concerns oscillating-armature electric motors, of the type typically used in electric dryshavers, and comprised of an armature mounted for swinging motion and a stator provided with a winding which is electrically energized, e.g., by alternating current, to impart swinging motion to the armature. In dryshavers, the armature is coupled to and vibrates the cutting block beneath the apertured cutting head of the shaver.
Oscillating-armature electric motors are most typically used where high electromechanical efficiency is required. Their high efficiency results from the fact that they are normally designed to have a mechanical-oscillation resonant frequency very close to the frequency of the energizing current employed.
Various types of oscillating-armature motors have been developed, some with and others without the use of auxiliary permanent magnets; see e.g., "Der Elektroniker," 1967, No. 5, pp. 251-255. These oscillating-armature motors tend to require relatively large stator windings, so that the space consumed by the motor itself may become not inconsiderable. Also, and in many instances more importantly, these motors, along with the high efficiency referred to above, nevertheless additionally exhibit very high levels of wasteful electrical power dissipation, i.e., power dissipation not significantly contributing to the development of mechanical driving force. Oscillating-armature motors specially designed to drive small dryshavers are likewise known; see e.g., W. Klenk, "Zur Theorie des Schwingankermotors fur Elektrorasierer," Diss. Univ. (TH) Stuttgart, 1971, page 96. However these, in the same sense, tend to require relatively large stator windings and exhibit rather high levels of wasteful power dissipation.
The Philips Corporation "Technische Rundschau," 33, 1973/1974 discloses oscillating-armature motors, with and without the use of permanent magnets, designed for use in refrigerator compressors and in dryshavers, and in FIG. 7 on page 262 of that publication, an informative graph is provided for a motor intended for dryshaver use, showing the variation with respect to time of the position of the oscillating armature, of the energizing current, of the motor flux and of the electromagnetic force produced. If one compares the time-variation of armature position against the time-variation of energizing current, it becomes apparent that energizing current is very much flowing even when the magnetic poles of the swinging armature are located very remote from the magnetic poles of the stator. This is disadvantageous because, due to the high magnetic resistance presented by the very sizable air gap which forms when the armature poles are remote from the stator poles, the magnetic field resulting from electrical energization is actually highly effective for the production of electromagnetic force only during about one third of the armature's swing. During the remaining two thirds or so of the armature's swing, the electrical energy furnished to the motor is mainly converted into heat.
Attempts to reduce the amount of wasteful power dissipation have already been made. For example, in the publication by Adolf Wilhelm Mohr, "Uber die gunstigste Gestaltung von Schwinganker-Rasiermotoren," ETZ-A, Volume 82, Book 26 of Dec. 18, 1961, pp. 852-855, this is attempted by designing the field versus armature-position profile in a very special manner. The profile there considered best is shown in FIG. 7 of that article and is partly straight-line and partly hyperbolic in configuration; i.e., the air gap varies with armature position at first linearly and then has a hyperbolic merge in the constant air gap. With this so-called best field profile there nevertheless continues to be a relatively high level of wasteful power dissipation during the part of the armature swing in which the armature is located remote from the stator.
German published patent application 1,488,056 discloses an oscillating-armature electric motor in which the amplitude of the armature swing is maintained constant independently of the mechanical load applied to the motor and independently of the driving voltage applied to the motor. In this motor, the magnetic pole surfaces are of different sizes proceeding in the direction of armature movement. However, this expedient does not actually serve to substantially reduce the wasteful power dissipation in question.
It should be noted that the problem in question does not exist with all oscillating-armature electrical motors. For example, French Pat. Nos. 1,604,481 and 2,268,386 disclose motors in which the armature has two magnetic poles but the stator has three. The latter are so disposed that, throughout the armature swing, the two magnetic poles of the armature are close to the first and middle pole of the stator or else to the middle and third pole of the stator. The stator winding is constantly connected to voltage.
German published allowed patent application 1,262,431 describes an oscillating-armature electric motor in which the motor is not constantly energized, because the motor is to be operated at an oscillatory frequency different from the available line frequency, e.g., an oscillatory frequency of 25 Hz derived from an available line frequency of 50 or 60 Hz. With the motor-energization circuit there disclosed, if the mechanical oscillatory frequency is to be, e.g., one-half the line frequency, then the motor is energized only during every second A.C. voltage cycle, and is unenergized during the intervening A.C. voltage cycles. There is no particular relationship or correlation between the energization phases of the motor, on the one hand, and the phases of the armature's swing and of the armature's distance from the state, on the other hand.